Subtropical species

Palaeogeography, Palaeoclimatology, Palaeoecology 223 (2005) 49 – 65 www.elsevier.com/locate/palaeo

The biogeography of major diatom taxa in Southern Ocean surface sediments: 3. Tropical/Subtropical species O.E. Romeroa,T, L.K. Armandb,1, X. Crostac, J.-J. Pichonc,F a

Department of Geosciences and RCOM, University of Bremen, PO Box 330440, 28334 Bremen, Germany b School of Earth Sciences, University of Tasmania, GPO Box 252-79, Hobart, Tasmania 7001, Australia c DGO, UMR-CNRS 5805 EPOC, Universite de Bordeaux I, Avenue des Facultes, 33405 Talence Cedex, France Received 22 November 2004; accepted 24 March 2005

Abstract This paper gives a modern circumscription of Tropical/Subtropical diatoms regarding their relationship with sea-surface temperatures (SST) and sea ice cover. Diatoms from 228 core-top sediment samples collected from the Southern Ocean were studied to determine the geographic distribution of eight major diatom species/taxa preserved in surface sediments generally located north of the Subantarctic Front. The comparison of the relative contribution of diatom species with modern February SST and sea-ice cover reveals species-specific sedimentary distributions regulated both by water temperatures and sea ice conditions. Although selective preservation might have played some role, their presence in surface and downcore sediments from the Southern Ocean are reliable indicators of high SST and poleward transport of waters from the Tropical/Subtropical Atlantic. Our work supports the use of diatom remains to reconstruct past variations of these environmental parameters via qualitative and transfer function approaches. D 2005 Elsevier B.V. All rights reserved. Keywords: Diatom; Bacillariophyceae; Biogeography; Sea ice; Southern Ocean; Sediments

1. Introduction

T Corresponding author. E-mail addresses: [email protected] (O.E. Romero), [email protected] (L.K. Armand), [email protected] (X. Crosta). 1 Current address: Centre d’Oce´anologie de Marseille, Laboratoire d’Oce´anologie et de Bioge´ochemie, Campus de Luminy, case 901, F-13288 Marseille Cedex 09, France. F Deceased. 0031-0182/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2005.03.027

A substantial part of the ocean’s primary productivity is provided by diatoms (Tre´guer et al., 1995). As a general statement we may say that diatoms dominate primary production in a number of oceanographic settings that offer both the high nutrient and turbulence conditions (Nelson et al., 1995). Biogenic silica (opal) production and export, mostly provided by diatoms, are also crucial for the organic carbon fluxes to the deep sea (Ragueneau et al., 2000). In areas with

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present-day high opal flux such as south of the Polar Front and in some coastal upwelling systems (e.g. off Namibia), a linear relationship between biogenic silica and organic carbon (Corg) fluxes is seen (Fischer et al., 2002; Romero et al., 1999, 2003). Surface sediments with high opal content are found around the Antarctic continent, along the margins of eastern boundary currents, and in the equatorial Pacific. In particular, diatom assemblages preserved in sediments underlying Antarctic waters have been widely used for late Quaternary paleoceanographic studies such as sea-surface temperature paleoreconstructions (Pichon et al., 1992; Labeyrie et al., 1996) and estimations of past sea-ice cover (Armand, 1997, 2000; Armand and Leventer, 2003; Crosta et al., 1998a,b, 2004). Reconstructions of late Quaternary sea surface temperatures (Burckle, 1984; Pichon et al., 1992; Crosta et al., 2004) and sea ice extent (Crosta et al., 1998a,b) are mainly based on the assumption that fossil diatom assemblages mirror surface water hydrology. However, the relationship between surface water conditions and preserved diatom assemblages is partially known, mainly due to the fact that earlier works covered geographically reduced areas with uneven sample (Kozlova, 1966; Donahue, 1973; Semina, 1979; DeFelice and Wise, 1981). In addition, diatom taxonomy has experienced significant changes in the last three decades and the integration of new data sets allows improving our knowledge of the relationship between water column and sediment diatom signal. The Southern Ocean north of the Polar Front is one of the large areas of the world ocean where comparatively little data on diatom occurrence are available. The area of research is contained within the more frequently labeled Southern Ocean, the oceanic region from the coast of Antarctica (~708S) to the Subtropical Convergence (~408S) which acts as a connective pool to the world’s oceans (Gordon, 1971; Tchernia, 1980; Tomczak and Godfrey, 1994). Although large diatom data sets have been recently compiled for both the South Atlantic and Indian Oceans, and applied on transfer functions (Pichon et al., 1992; Zielinski and Gersonde, 1997), our knowledge of diatom distribution in the Southern Ocean is still incomplete. The relevance of mapping diatom distribution is important since one of the basic assumptions of transfer function work is that the

species preserved in sediments reflect the distribution of physical parameters of overlying surface waters. This paper represents one in a series of three papers describing the diatom distribution in surface sediments from wide areas of the Southern Ocean (Crosta et al., 2005; Armand et al., 2005). In the paper to hand, we present distribution maps of eight Tropical/ Subtropical diatom species or group of species thriving in 228 core-top surface sediment samples from the Southern Ocean. Our dataset is based on the original work by Pichon et al. (1992), recently improved for statistically based paleoreconstructions by Crosta et al. (1998a,b). Relative abundances of the eight diatom species or group of species from core-top samples are additionally compared to summer seasurface temperatures, and sea-ice duration and concentration in order to document the extent the preserved diatom record reflects Southern Ocean surface water hydrology and subsequently, the use of this environmental information for future palaeoenvironmental reconstructions.

2. Materials and methods Detailed methodology including database treatment, mapped surface sample locations and oceanographic boundaries, modern parameter sources and extraction for sea ice and sea surface temperature (SST) data are given in Armand et al. (2005, inclusive of Figs. 1–3). The reference dataset is composed of 228 core-top sediment samples derived from the Crosta et al. (1998a) dataset and unpublished counts from the South Tasman region (Armand, unpublished data) and the Subantarctic sector of the Atlantic Ocean (Romero, unpublished data). These samples are representative of modern to sub-modern climate conditions. The reference dataset will be hereafter referred to as the Diatom Database 228 (DD228). Taxonomic identification principally follow Fenner et al. (1976), Simonsen (1974), Fryxell and Hasle (1980), Fryxell et al. (1986), Kaczmarska and Fryxell (1996), Moreno-Ruiz and Licea (1996), and Hasle and Syvertsen (1996). We present in this paper the eight species or group of species that are linked to Tropical/ Subtropical water conditions. As tropical to subtropical diatoms we consider extant phytoplankton com-

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ponents, predominantly associated with low to midlatitudes, pelagic, saline, warm waters of the world ocean. Tropical/Subtropical diatoms usually occur in oligotrophic areas with low silicate content (annual average of silicate content b5 AM).

3. Observations and discussion The geographical distribution and abundance in surface sediments of Tropical/Subtropical diatoms are described and related to present-day sea-surface temperate (SST) and sea-ice cover. In addition, some taxonomical issues are addressed. Due to the detail required, but not possible to deliver in this work, the geographic positioning of samples in the Ross Sea, Prydz Bay area and Antarctic Peninsula region (Armand et al., 2005, Figs. 1 and 2) have been summarised by regional boxes in all distribution maps. Previous studies on the distribution of diatom species in surface sediments geographically cover large areas of the Southern Ocean and those that provide data on relative abundances relevant to this study are schematically summarized elsewhere (Armand et al., 2005, Fig. 3). Since this work mainly focuses on diatom distribution in relation with SST and sea-ice cover, more general reports on species distribution in sediments were not considered. 3.1. Alveus marinus (Grunow) Kaczmarska and G. Fryxell Figures in: Kaczmarska and Fryxell (1996), figs. 1–35. Alveus marinus is a cosmopolitan, Tropical/Subtropical diatom whose contemporary distribution spans ca. 408N to 408S. It has been found in equatorial oceans, the Mediterranean and Arabian Seas (Kaczmarska and Fryxell, 1996, and references therein; Semina, 2003). As for other heavily silicified diatoms, which are rare components of phytoplankton communities, A. marinus is more frequently found in sediment than in water samples. Common sediment occurrences of A. marinus are confined to the narrow equatorial band of siliceous bottom sediments in the Pacific and Indian Oceans (approximately 108N– 108S), in a broader band in the Atlantic (308N–

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358S, Kaczmarska and Fryxell, 1996; Romero et al., 2000; Semina, 2003), the eastern South Atlantic (Pokras and Molfino, 1986; Romero et al., 1999), and in the Argentina Basin (Romero and Hensen, 2002). The sedimentary fossil record of A. marinus goes back to Middle–Late Miocene (approximately 8 Ma, Kaczmarska and Fryxell, 1996). In relation to modern SST, a sharp increase in the relative contribution of Alveus marinus is seen north of the present-day position of the Subtropical Front in the Atlantic Ocean with highest abundance values (up to 24%) above 16 8C (Fig. 2). Sea-ice plays no role in the distribution of this species. Our observations agree with the previous record of Alveus marinus for the Southern Ocean: the southernmost distribution is limited by the Subantarctic Front (Fig. 2; Zielinski and Gersonde, 1997). The SST range in our study is slightly higher than the one presented earlier by Zielinski and Gersonde (1997): 15.5–20 8C, while the upper limit of A. marinus occurrence in our DD228 reaches up to ca. 23 8C (Table 1, Fig. 1). This small difference possibly derives from the fact that our dataset includes several samples from the South Atlantic north of 41–398S (the present-day position of the STF, Orsi et al., 1995). Hence, the record of A. marinus in surface sediments from the Southern Ocean clearly mirrors the occurrence of warm water masses, and is southwardly restrained by low SST and sea ice cover. Occurrences below 1% south of the Polar Front are considered a matter of southward bound water transport and selective preservation. 3.2. Azpeitia tabularis var. tabularis (Grunow) G. Fryxell and P.A. Sims Figures in: Hustedt (1930) fig. 230a; Fryxell et al. (1986) Pl. XXX, fig. 1. Azpeitia tabularis is a typically Tropical/Subtropical diatom, presently found in all the oceans, including Subantarctic areas (Hasle and Syvertsen, 1996). Some taxonomic confusion is associated with this centric diatom. Hustedt (1930) already recognized certain degree of morphological variation and defined the variety egregius, formerly Coscinodiscus tabularis var. egregius, mainly distinguished by a denser areolation, a reduced marginal hyaline ring, and its lack of a central hyaline area. Although Fryxell et al.

(d)

September Sea Ice Concentration (% of covered water)

(c)

February Sea Ice Concentration (% of covered water)

Sea Ice Duration (Months per year)

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io

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February Sea Surface Temperatures (˚C)

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T. n vaitzs r. ch pa io rv ide a s

O.E. Romero et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 223 (2005) 49–65

is

52

30

0 1 2 3 4 5 6 7 8 9 10 11 12 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90

100

Fig. 1. Relative abundance of the eight major Tropical/Subtropical diatom species and/or group of species in DD228 plotted against (a) February sea-surface temperature, (b) sea ice duration, (c) February sea-ice concentration, and (d) September sea-ice concentration.

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24

27 9

17

A. marinus 0 - 1% 1 - 2% 2 - 5% 5 - 10% 10 - 20% 20 - 30%

Fig. 2. Distribution of Alveus marinus relative abundance in DD228. Location of the oceanic fronts from Orsi et al. (1995), and location of February and September maximum sea ice extent from Comiso (pers. comm., 2002).

(1986) described only the nominate variety; they also stated that A. tabularis has a wide range of morphological variation as well as cold and warmwater forms. However, no relation between morphological variation and temperature spatial pattern was offered by Fryxell et al. (1986). Fenner et al. (1976) concluded that both varieties, tabularis and egregius, share the same spatial distribution in Subantarctic waters. Armand (1997) did not find any particular spatial trends for both varieties in her database counts, although samples available were few. Since there seems to be no further information on a separation of cold and warm water forms, counts of A. tabularis and A. tabularis var. egregius were pooled together in our DD228. This variety difference may indicate seasonal variability morphology that could be investigated further by culture experimentation. Azpeitia tabularis shows its highest contribution north of the Polar Front in sediments of the Southern Ocean (Fig. 3). A clear south–north increase is

observed. Maximum abundances up to ca. 24% are located in warm SST’s (13 8C February, 11 8C August; Table 1, Fig. 1). Sea-ice plays only a secondary role in its distribution, since occurrences below 5% are observed during September sea ice concentration (Fig. 1). Perceived inconsistencies in Azpeitia tabularis’ ecological affinities may derive from the fact that the varieties tabularis and egregius are usually counted as one entity. In surface sediments from the South Atlantic north of the STF, solely the nominate variety has been found (Romero et al., 1999; Romero and Hensen, 2002; Romero, unpubl. obs.). Our observations on combined varieties agree well with those presented by Zielinski and Gersonde (1997), where maximum abundances correlate strongly with higher SST (Fig. 3). The combined varieties can be classified as cold-tolerant in Southern Ocean waters but clear preferences for Tropical/Subtropical conditions are apparent. As observed previously by Fryxell et al. (1986), A.

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25

26 10

17

Az. tabularis 120°W0

- 1% 1 - 2% 2 - 5% 5 - 10% 10 - 20% 20 - 30%

Fig. 3. Distribution of Azpeitia tabularis relative abundance in DD228. Location of the oceanic fronts from Orsi et al. (1995), and location of February and September maximum sea ice extent from Comiso (pers. comm., 2002).

tabularis is uncommon near the sea-ice, but is generally confined to Subantarctic and Antarctic waters (Fig. 3). Zielinski and Gersonde (1997) also suggested Table 1 Temperature ranges and maximum relative abundances of major Tropical/Subtropical diatom species and/or species groups in DD228 Taxa/Taxa group

Temp. range (8C)

Max. Temp. occurrence of Max. (%) occurrence (8C)

Alveus marinus Azpeitia tabularis gp Fragilariopsis doliolus Hemidiscus cuneiformis Roperia tesselata Thalassionema nitzschioides var. parva Thalassiosira symmetrica gp Thalassiosira oestrupii gp

12 to 22.3 11 to 22.3 10.5 to 22.3 8.5 to 22.3 6.5 to 22.3 8.5 to 20

24 23.2 33.3 11.2 13 2.9

N17 11 to 13.5 17 to 18 11 to 19 N11 ~16

6.5 to 22 4.5 to 22.3

7.9 21

11 to 13 12 to 19

that the species occurrence is southward restricted by the maximum winter sea-ice edge. Occurrences below 1% along the Antarctic coast have been reported by several authors (Truesdale and Kellogg, 1979; Stockwell et al., 1991; Tanimura, 1992; Leventer, 1992; Cunningham and Leventer, 1998). 3.3. Fragilariopsis doliolus (Wall.) Medlin and Sims Figures in: Hustedt (1958) Pl. 12, figs 147; Medlin and Sims (1993) figs 6, 9. Fragilariopsis doliolus is the only Fragilariopsis species with Tropical/Subtropical distribution (Medlin and Sims, 1993; Hasle and Syvertsen, 1996; Semina, 2003). It is a common component of extant diatom assemblages in Tropical/Subtropical areas of the Atlantic, Pacific, and Indian Oceans, as well as in Mediterranean and Arabian Seas (Simonsen, 1974). For a wide area of the Atlantic Ocean, F. doliolus has been characterized as thriving in tropical to temperate

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reported abundance in the Southern Ocean is ~29% at N15 8C (Zielinski and Gersonde, 1997). Surface sediment studies further northward in the SE Atlantic and Pacific have observed similarly high abundances (e.g. ~20% Pokras and Molfino, 1986, max. 37%, Schuette and Schrader, 1981; ~14% off Peru, Schuette and Schrader, 1979a,b). In core-top sediments from the Argentina Basin, occurrence of F. doliolus is associated with the main path of southward-flowing, tropical water masses (Romero and Hensen, 2002).

regimes of moderate-to-low production (Pokras and Molfino, 1986; Romero et al., 2000, 2001; Romero and Hensen, 2002). The strongly silicified, semilunate valves of F. doliolus are easily preserved in surface and downcore sediments. The occurrence of Fragilariopsis doliolus in our DD228 is confined to the north of the Subantarctic Front (SAF; Fig. 4). An abrupt abundance decrease is seen above ~13 8C with relative values increasing with highest February SST (Fig. 1). Maximum abundances are found in surface sediments underlying SST warmer than 7 8C and at SST maxima of 13 8C and 11 8C in February and August, respectively. Distribution of F. doliolus in surface sediments shows no relationship with sea-ice cover (Fig. 1). The SAF marks the southernmost limit of Fragilariopsis doliolus in core-top sediments (Fig. 4). Our observations coincide with information presented by earlier works (Hasle, 1976; Hasle and Syvertsen, 1996; Zielinski and Gersonde, 1997). Maximum

3.4. Hemidiscus cuneiformis Wallich Figures in: Hustedt (1930) fig. 542; Fryxell et al. (1986), fig. XXVI. Hemidiscus cuneiformis is a typically warm-water diatom (Hasle and Syvertsen, 1996; Semina, 2003). It never occurs in high numbers, and has been observed as rare component of diatom assemblages in the Arabian Sea and Indian Ocean (Simonsen, 1974),

24 27

10

F. doliolus

17

0 - 1% 1 - 2% 2 - 5% 5 - 10% 10 - 20% 20 - 30% 30 - 50%

Fig. 4. Distribution of Fragilariopsis doliolus relative abundance in DD228. Location of the oceanic fronts from Orsi et al. (1995), and location of February and September maximum sea ice extent from Comiso (pers. comm., 2002).

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DDT228 may result from our inclusion of surface sediments below the South Atlantic Gyre (Fig. 5). Other South Atlantic studies confirm the presence of H. cuneiformis in sediments under warm-water influence (Schuette and Schrader, 1981; Romero and Hensen, 2002). Along the southeast and southwest South American coast, H. cuneiformis is a rare component of the diatom thanatocoenosis (Schuette and Schrader, 1979a,b; Romero and Hensen, 2002; Romero and Hebbeln, 2003). Similar reports exist from the SE Indian Ocean (Abbott, 1973). In the Pacific sector, Donahue (1973) found the species sporadically north of the Polar Front with maximum abundance of 7%.

and sediment traps from the Atlantic Ocean south of the equator (Romero, unpublished). The species has been found in sediments underlying warm-temperate waters from the Pacific, Indic and Atlantic Oceans (Fryxell et al., 1986, and references therein; Romero, unpublished). Occurrence of Hemidiscus cuneiformis in temperate-to-warm waters is reflected by the relationship observed with both February and September SST: highest abundances associated with N11 8C SST (Fig. 1). Occurrence of H. cuneiformis is not related with sea-ice cover. Our work confirms previous distribution records of Hemidiscus cuneiformis in surface sediments (Fig. 5). Zielinski and Gersonde (1997) reported abundances up to ~7% in austral summer SST N15 8C, yet the species was found to occur over a temperature range of 7.5–20 8C in the South Atlantic. Slight differences in February SST and H. cuneiformis relative contributions between Zielinski and Gersonde’s observations and our

3.5. Roperia tesselata Grunow ex Pelletan Figures in: Hustedt (1930) fig. 297: Fryxell et al. (1986) fig. XXXII, no. 4a–b. The discoid cells of Roperia tesselata primarily prefer warm waters with moderate to low productivity

24

25 10

H. cuneiformis 17

0 - 1% 1 - 2% 2 - 5% 5 - 10% 10 - 20%

Fig. 5. Distribution of Hemidiscus cuneiformis relative abundance in DD228. Location of the oceanic fronts from Orsi et al. (1995), and location of February and September maximum sea ice extent from Comiso (pers. comm., 2002).

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SST range given by Zielinski and Gersonde (1997) is slightly different from the present observations: R. tesselata is associated with a colder range of SST than other typical Tropical/Subtropical diatoms (Table 1, Fig. 1). Higher relative abundances of R. tesselata in DD228 than those found by Zielinski and Gersonde (1997) are probably due to the fact that our data set includes additional northern locations and a greater number of samples. In our DD228 R. tesselata shows no occurrences southward of the Subantarctic Front.

(Simonsen, 1974; Hasle and Syvertsen, 1996; Semina, 2003). It has been observed as permanent component of sediment trap-collected diatom associations in the tropical and subtropical Atlantic and SW Pacific Ocean (Romero et al., 2001, 2002, 2003). In DD228, occurrence of Roperia tesselata is restricted northward of the SAF (Fig. 6). Highest relative abundances (~13%) are seen north the STF in the Atlantic Ocean. Maximum abundances occur in surface sediments underlying waters with a SST range of 7–14 8C (Fig. 1). No relationship is seen between sea-ice cover and relative abundances of R. tesselata. In good agreement with water-column studies, Roperia tesselata appears in surface sediments from tropical and subtropical areas. Roperia tesselata has been recorded throughout the South Atlantic (DeFelice and Wise, 1981; Schuette and Schrader, 1981; Zielinski and Gersonde, 1997; Romero et al., 1999; Romero and Hensen, 2002) and up to the equator (Pokras and Molfino, 1986; Romero et al., 2000). The

3.6. Thalassionema nitzschioides var. parva Heiden and Kolbe Figures in: Moreno-Ruiz and Licea (1996), figs. 25–27, 57–58. Thalassionema nitzschioides possesses a high amount of morphologically variable varieties covering a wide range of ecological regions of the world ocean. These varieties basically differ in their length, width

24

27 9

R. tesselata

17

0 - 1% 1 - 2% 2 - 5% 5 - 10% 10 - 20%

Fig. 6. Distribution of Roperia tesselata relative abundance in DD228. Location of the oceanic fronts from Orsi et al. (1995), and location of February and September maximum sea ice extent from Comiso (pers. comm., 2002).

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during the last 20 years (Hallegraeff, 1986; MorenoRuiz and Licea, 1996; Hasle, 2001), it is possible that the occurrence of some varieties in surface sediments from the Southern Ocean have been overlooked. Besides the varieties mentioned above, possibly T. nitzschioides var. claviformis and T. incurvata (sensu Moreno-Ruiz and Licea, 1996) have also been observed in a few samples. Our DD228 includes the nominate species and several other varieties. We describe here only the distribution of T. nitzschioides var. parva as a typically oligotrophic, warm-water variety and follow the description presented by Moreno-Ruiz and Licea (1996). Occurrence of T. nitzschioides var. nitzschioides is described by Crosta et al. (2005). Thalassionema nitzschioides var. parva is a secondary component of the Tropical/Subtropical diatom assemblage where its relative abundance by and large remains below 2% throughout the study area (Table 1, Fig. 7). South of the SAF, abundances never exceed

and the heteropolarity/isopolarity of the valves. Hallegraeff (1986) and Hasle (2001) examine the morphology, taxonomy, nomenclature, and distribution of recent species. Moreno-Ruiz and Licea (1996) describe 10 varieties of T. nitzschioides mainly based on the valve polarity and the areola structure. They conclude that the valve outline is a reliable feature for distinguishing T. nitzschioides varieties. Several varieties of Thalassionema nitzschioides have been reported from the Southern Ocean: the cosmopolitan var. nitzschioides (Crosta et al., 2005, and references therein), var. capitulata and var. lanceolata (Hustedt, 1958; Moreno-Ruiz and Licea, 1996; Zielinski and Gersonde, 1997); and var. parva, a warm-water taxon whose southernmost occurrence is associated with the STF (Zielinski and Gersonde, 1997). The cold-water form T. nitzschioides forma 1 is clearly associated with SST between 0 and 3 8C (Zielinski, 1993; Zielinski and Gersonde, 1997). Since the taxonomy has undergone important changes

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26 10

T. nizschioides var parva

17

0 - 1% 1 - 2% 2 - 5%

Fig. 7. Distribution of Thalassionema nitzschioides var. parva relative abundance in DD228. Location of the oceanic fronts from Orsi et al. (1995), and location of February and September maximum sea ice extent from Comiso (pers. comm., 2002).

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future research should consider the varieties as separated entities, since they are important oceanographic indicators.

1% and only in the Indian Ocean do values raise over 2% north of STF. Being a Tropical/Subtropical species, T. nitzschioides var. parva’s occurrence in surface sediments clearly reflects summer conditions where the SST range is 11–19 8C (Fig. 1). Aside from trace occurrences in sea-ice covered regions that may be referable to T. nitzschioides forma 1 (sensu Zielinski and Gersonde, 1997), this diatom is not associated with sea-ice cover. In good agreement with previous records from warm to temperate ocean areas, T. nitzschioides var. parva appears in DD228 as a secondary contributor to the diatom thanatocoenosis underlying high salinity, moderate-to-low nutrient waters (Schuette and Schrader, 1979a,b, 1981; Pokras and Molfino, 1986; Zielinski and Gersonde, 1997; Romero and Hensen, 2002; Romero et al., 2003). The wide morphological variability of T. nitzschioides varieties complicates its identification, and hence, its use in paleoceanographic studies should be considered carefully. Nevertheless, we strongly recommend that

3.7. Thalassiosira symmetrica Fryxell and Hasle and morphologically related species Figures in: Fryxell and Hasle (1972), pl. IX, figs 38–40; pl. X, figs 41–46. This minor group of DD228 (~2%) includes several species with eccentric areolation arranged around a heptagonal central areola such as Thalassiosira symmetrica (Fryxell and Hasle, 1972). Although T. symmetrica has not been seen in chains, it is morphologically related to T. eccentrica, T. tumida and Planktoniella sol (review in Fryxell and Hasle, 1972). In spite of on-going taxonomic correction to the DD228 data set we acknowledge that the majority of taxa reported in this group are cosmopolitan and cover a wide SST range as expanded on below.

25

27 10

17

T. symmetrica group 0 - 1% 1 - 2% 2 - 5%

Fig. 8. Distribution of Thalassiosira symmetrica group relative abundance in DD228. Location of the oceanic fronts from Orsi et al. (1995), and location of February and September maximum sea ice extent from Comiso (pers. comm., 2002).

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abundances (26.5%) were observed by Schuette and Schrader (1979a,b) in the southeast Pacific off Peru.

Distribution of the DD228 T. symmetrica group indicates a slight trend of northwardly increasing abundances over a broad SST distribution range (Fig. 8). The relative abundances interpreted in terms of February and September SST show major contributions in warm waters (11–14 8C Feb. SST; Table 1, Fig. 1). Sea-ice plays no role in the distribution of this species group. Thalassiosira symmetrica and associated diatoms– mainly T. eccentrica and P. sol–are typically Tropical/ Subtropical species (Hasle and Syvertsen, 1996). Fryxell and Hasle (1972) describe T. symmetrica as a widely distributed oceanic species from the equatorial regions through the southern temperate latitudes. The presence of T. symmetrica has not been reported south of the STF (Johansen and Fryxell, 1985). Nevertheless, some sediment records exist from Ross and Amundsen Seas (Truesdale and Kellogg, 1979; DeFelice and Wise, 1981; Akiba, 1982; Kellogg and Kellogg, 1987; Andreoli et al., 1995). Highest relative

3.8. Thalassiosira oestrupii var. oestrupii (Ostenfeld) Hasle Figures in: Fryxell and Hasle (1980) figs 1–3, 6–7, 12–19 (variations oestrupii and venrickae). Thalassiosira oestrupi var. oestrupii is a simply identifiable diatom. Two varieties are so far defined, var. oestrupii and var. venrickae, with overlapping distribution in low latitudes (Fryxell and Hasle, 1980; Semina, 2003). Although cosmopolitan in its distribution and predominantly tropical to temperate in its temperature preferences, the species has been recorded as rare in waters as far as north as the Norwegian See and as far as south as the Wedell Sea (Fryxell and Hasle, 1980). Usually well-preserved in surface and downcore sediments, T. oestrupii never becomes dominant, acting rather as an accompanying

24 27 10

T. oestrupii

17

0 - 1% 1 - 2% 2 - 5% 5 - 10% 10 - 20%

Fig. 9. Distribution of Thalassiosira oestrupii var. oestrupii relative abundance in DD228. Location of the oceanic fronts from Orsi et al. (1995), and location of February and September maximum sea ice extent from Comiso (pers. comm., 2002).

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species of tropical to temperate assemblages of the world ocean (Hasle and Syvertsen, 1996). In this work both varieties were plotted together. Relative contribution of T. oestrupii decreases southward. Highest relative contribution up to ~5% is recorded only in the Atlantic Ocean north of the SAF, while only trace occurrences are observed in the Indian Ocean (Fig. 9). No relationship is seen between T. oestrupii and sea-ice cover. As for most of the Tropical/Subtropical diatoms, contribution of T. oestrupii sharply increases at 11 8C. The pelagic Thalassiosira oestrupii has been widely recorded in net hauls collected in the Pacific and North Atlantic oceans (Fryxell and Hasle, 1980). On their account on Thalassiosira spp. in the Antarctic, Johansen and Fryxell (1985) stated that T. oestrupii is uncommon south of the Antarctic Convergence Zone. This observation coincides with occurrence pattern of T. oestrupii in our core-top samples: only north of SAF we note relative abundances greater than 1%, with the PF constraining the southernmost limit (Fig. 9; see also DeFelice and Wise, 1981; Zielinski and Gersonde, 1997). Highest contributions are seen north of the STF in both Atlantic and Indian Oceans. Trace occurrence south of the PF is probably explained by the highly silicified valves and its selective preservation in sediments. Zielinski and Gersonde (1997) give a wider SST range (5–20 8C) than our observations here.

4. Discussion 4.1. Diatom spatial pattern in the southern ocean and ecological conditions Our survey of 228 core-top samples collected from a wide area between coastal waters off Antarctica and the Subantarctic Atlantic and Indian Oceans provides a modern sediment-based biogeography for diatoms preserved in surface sediments. In comparison with earlier, regionally more constrained studies (DeFelice and Wise, 1981; Zielinski and Gersonde, 1997), our dataset DD228 covers a larger geographic area, and additionally describes the relationships between diatom abundance pattern, and sea-ice cover and summer SST data. Hence, the present work reviews more comprehensively the relationships between diatom

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occurrence and environmental conditions. This information will help to reliably interpret the downcore diatom signal in future diatom-based paleoceanographic in the Southern Ocean. It should be kept in mind that comparison of diatom distribution and abundance reported with previous published accounts include some differences in (a) preparation and counting methods, and (b) the taxonomic concept of each identified species (mainly with DeFelice and Wise, 1981). Some misidentifications possibly occurred in our DD228 dataset in the past because incomplete descriptions and illustrations. In spite of these restrictions, a general good agreement is seen in the distribution pattern of Tropical/ Subtropical diatoms in surface sediments between our and previous reviews (mainly Zielinski and Gersonde, 1997, for the Atlantic sector of the Southern Ocean). In plotting the environmental data against diatom relative abundances we show the strong relationship of each species to SST. The Tropical/Subtropical diatom species are less influenced by sea-ice cover (Fig. 1). This shows that the Tropical/Subtropical group can be reliably used in future reconstructions of past oceanographic conditions regarding SST and sea ice cover. In spite of some above-mentioned taxonomic uncertainties, the Tropical/Subtropical diatom group accurately represents oligotrophic, warm-water conditions typical for the South Atlantic Gyre. The abrupt increase of the Tropical/Subtropical diatom assemblage coinciding with the 11 8C isoline reveals the clear preference of this diatom group for temperate to tropical waters. Hemidiscus cuneiformis, Roperia tesselata, Thalassionema nitzschioides var. parva, and Thalassiosira oestrupii are all diatoms typically thriving in oligotrophic, warm, saline waters and can be used as good indicators of the southward migration of warmer, nutrient-poorer water masses into the Southern Ocean. Their lowermost tolerance limit in surface sediments is uniformly 11 8C which represents the average SST associated with the present-day STF (Orsi et al., 1995; Figs. 5–8). The contribution of Alveus marinus and Fragilariopsis doliolus abruptly increases at 16 8C. Both diatoms dominate the assemblage in surface sediments underlying waters up to ~22 8C (Hasle and Syvertsen, 1996). Although selective preservation might have played some role, their presence in surface and

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downcore sediments from the Southern Ocean are reliable indicators of high SST and poleward transport of waters from the Tropical/Subtropical Atlantic. Azpeitia tabularis represents a special case. Regarding this well-silicified diatom one aspect to be considered is its morphological variability. In our earlier dataset, no clear distinction was made between the nominate variety and var. egregius. Since most of the authors did not distinguish between varieties tabularis and egregius in their countings, it is rather difficult to state whether geographical preferences exist. It is not only a matter of taxonomy and diatom morphology but most significantly a matter of (apparent) ecological preferences. Azpeitia tabularis var. tabularis seems to be a cosmopolitan diatom with a wide range of temperature tolerance, although it’s mainly restricted to tropical and Subantarctic waters. The species has been characterized as the only Azpeitia with a preference for cold waters (Fenner et al., 1976; Fryxell et al., 1986; Hasle and Syvertsen, 1996; Semina, 2003). Its contribution in sediment trap–collected assemblage from the Southern Ocean is, however, negligible (Fischer et al., 2002). The variety egregius has the same distribution as the species, without being abundant (Fenner et al., 1976). More recent studies from tropical and subtropical areas of the Atlantic ocean describe the variety tabularis as a tropical-to-subtropical diatom without mentioning the occurrence of var. egregius (Pokras and Molfino, 1986; Romero et al., 1999, 2002; Romero and Hensen, 2002). In our DD228 both varieties occur, the nominate being always dominant. In good agreement with most of the Tropical/ Subtropical species, 11 8C, the summer SST, appears as the abrupt southward distribution limit of the nominate variety in surface sediments. However, A. tabularis shows abundances lower than 10% below 11 8C, and is the only Tropical/Subtropical species whose occurrence is influenced by sea-ice duration and sea ice cover (Figs. 1 and 3). Cell size, silicification of the diatom frustule and the species composition of the diatom assemblage play an important role in determining whether the siliceous material of a given species will reach the seafloor and preserve in the sedimentary record (Nelson et al., 1995). The specific composition of the sediment diatom assemblage seems to be a consequence of strong dissolution of the fragile, most

abundant diatoms, and marked enrichment of robust species. All of the Tropical/Subtropical species considered in this work are moderately to heavily silicified. Although we lack information on plankton or sediment trap samples from the Subantarctic Atlantic and Indian oceans, sediment trap observations from other pelagic, Tropical/Subtropical, saline areas show that slightly silicified diatoms dominate in the water column during times of high production but do not preserve in the surface sediments (Romero et al., 2000, 2002). Hence, the effect of preservation usually removes information from the most productive season in Tropical/Subtropical areas, leaving the sediment assemblage enriched in more strongly silicified diatoms (Romero et al., 1999). Hence, on interpreting the Subantarctic diatom signal in surface and downcore sediments caution is advised. 4.2. Diatom previous records/taxonomic issues The taxonomy and systematic of diatoms have undergone dramatic changes in the last ca. 30 years. Almost each diatom genus treated in this work has experienced substantial taxonomic changes (e.g. Azpeitia, Fryxell et al., 1986; Fragilariopsis, Medlin and Sims, 1993; several Thalassiosira spp.). Several reviews have not only clarified the taxonomy of Tropical/Subtropical diatoms and described their frustule morphology, but they roughly outlined their geographic distribution. The work carried out by taxonomists has greatly helped the micropaleontological investigation. In providing this diatom sedimentary distribution assessment, our work reveals evident problems faced by researchers in this region regarding the need for careful taxonomic identification for some diatom species. Although the observed relationships between diatom species, and SST and sea ice cover give us confidence on applying diatom transfer functions for estimating past sea-surface conditions, we are aware that due to the bmixed conceptQ used for some species, our DD228 has still some limitations in its application. Misidentification of cold-water species as warm-water components (e.g. Azpeitia tabularis and Thalassionema nitzschioides varieties) makes difficult future works and understandings of SST and paleoceanographic changes, and leads to erroneous transfer function estimations in downcore analyses. This study

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is based on the original database of Pichon et al. (1992) in which several diatom species were misidentified (Armand et al., 2005). Although Pichon’s database has been recounted following an up-dated taxonomy and successfully used to reconstruct sea-ice extent during the Last Glacial Maximum (Crosta et al., 1998a,b) and over the last 200,000 years (Crosta et al., 2004), we suggest that more work is still necessary on A. tabularis, T. nitzschioides varieties, and Thalassiosira eccentrica and associated species due to the integration of different data sets. In the early 1970s, Simonsen (1974) proposed that one of the great difficulties in diatom studies was the lack of modern biogeography. Although diatom research has intensely developed during the last three decades, advances primarily relate to the frustule structure and physiology rather than species biogeography. Much more work is needed to be done in this direction. This paper gives a modern circumscription of Tropical/Subtropical diatom species regarding its relationship with SST and sea ice cover. For some marine planktonic diatoms their range of variability is not clearly constrained, taxonomy uncertainties exist and certain ecological uncertainties have to be added. Hence, the assessment of our DD228 arises issues on taxonomic identification that requires further work in order to more accurately constrain the ecological range of some Tropical/Subtropical diatoms, and its use as proxy for paleoceanographic reconstruction in the Southern Ocean.

Acknowledgments We gratefully acknowledge L. Burckle (LamontDoherty Earth Observatory), D. Cassidy (formerly of FSU Antarctic Research Facility), and R. Gersonde (Alfred-Wegener-Institute), A. Leventer (Colgate University) and F. Taylor (IASOS, University of Tasmania) for their generous contribution with samples and/or unpublished data. Discussions with U. Zielinski, R. Gersonde, L. Burckle and A. Leventer during our research aided the development of this manuscript. We also thank Brian Harrold (Australian National University, Canberra) Rick Smith, Lisette Robertson and Nerida Bleakley (ACE CRC, Hobart) for help in drafting the maps. Amy Leventer and an anonymous reviewer provided

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helpful reviews of the manuscript. L.A.’s research and preparation of the manuscript was funded by an Australian Postgraduate Research Award (ANU, Canberra, Australia) and an Australian Research Council Postdoctoral Fellowship (F39800347) at the Institute of Antarctic and Southern Ocean Studies and the School of Earth Sciences at the University of Tasmania. X.C. and J.-J.P.’s research is funded by CNRS (Centre National de la Recherche Scientifique), PNEDC (Programme National d’Etude de la Dynamique du Climat), and Missions Scientifiques des Terres Australes et Antarctiques Franc¸ aises (IFRTP-TAAF). O.R.’s research and preparation of the manuscript was funded by the SFB261 at the University of Bremen.

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